In connection with oil and gas producing installations it is well known that rapid cooling of the production fluid during normal production and particularly during temporary interruption of the production may result in the formation of hydrates, which may cause clogging of pipes and pipe connections. To delay cooling of the production fluid in case of an interruption of the production, some form of thermal insulation and heat storage medium has to be provided to the element through which the production fluid flows. Said element could for instance be a pipe, a manifold, a valve, a connector etc. WO 01/63088A1 and WO 2006/106406A1 disclose the use of a so-called heat bank for thermally insulating one or more elements included in a subsea installation. The heat bank comprises a casing, which is arranged to enclose a fluid having heat-storing capacity, for instance sea water, and which has an internal space for receiving said element or elements and said fluid with the fluid surrounding the respective element so as to allow the fluid to delay cooling of the element by means of heat stored in the fluid. Thus, by means of heat stored in the fluid inside the casing, the heat bank protects the respective element from cooling too rapidly. The fluid in the heat bank is heated by heat emitted from the protected element or elements during normal operation.
When a heat bank, or any other arrangement intended to contain fluid enclosed in a casing, is lowered into the sea, the casing will be subjected to an external load caused by the hydrostatic pressure of the surrounding sea water. The hydrostatic pressure and thereby the external load on the casing will gradually increase as the depth increases. In order to prevent the casing from collapsing at greater sea depths due to this external load, the pressure of the fluid inside the casing has to be balanced against the ambient sea water pressure by means of a pressure balancing device. There is a need for a simple and reliable pressure balancing device that is suitable for use in a heat bank or any other subsea arrangement that is to be lowered into the sea.